1. Field of the Invention
The present invention relates to an electrostatic transducer with a diaphragm supported by means of a stretching or clamping ring, and a back plate which is formed by a thin metal plate supported on a carrier member and mounted spaced apart from the diaphragm. The invention further relates to a method of manufacturing an electrostatic transducer and to a device for carrying out this method.
2. Description of the Prior Art
German Offenlegungsschrift No. 33 25 966 describes an electrostatic transducer whose back plate is constructed as a thin plate. In the assembled state of such a transducer composed of diaphragm, diaphragm support ring, spacer ring and back plate, a pressure is generally exerted on the back plate by means of a spring. In this type of arrangement, the back plate must be sufficiently stiff, so that the pressure of the spring does not twist or bend the back plate. Thus, the plate used as the back plate must have a certain minimum thickness. Any waviness present in the plate must be eliminated by a planing process prior to the assembly of the transducer.
In an electrostatic transducer of the above-described type, a plane parallel spacing of approximately 50 .mu.m must exist between the back plate and the diaphragm surface. Therefore, the planeness of the back plate must meet the highest requirements. These requirements can only be met by using a complicated manufacturing process for the back plate and by providing the back plate with the appropriate stiffness. It may be necessary for acoustic reasons to provide the back plate with holes. This results in an additional weakening of the material of the back plates.
Austrian patent No. 323,257 and U.S. Pat. Nos. 4,429,193 and 4,434,327 disclose electrostatic transducers with support and stop elements. However, as in the transducer described above, these transducers have a solid base plate which is provided with support elements for supporting the diaphragm of the transducer. As shown in Austrian patent No. 322,257, the support elements may be lower than the distance between diaphragm and the base plate, so that sufficient space is provided during normal acoustic irradiation of the microphone to permit the diaphragm to vibrate unimpeded, while contact of the diaphragm with the back plate is prevented during the occurrence of extremely high sound pressures.
German Offenlegungsschrift No. 31 07 282 shows that the back plate of an electrostatic transducer can also consist of a metal surface applied onto an oscillating plate. A plurality of through openings in this insulating plate ensure that the sound permeability of the plate is sufficient. In this case, the insulating plate is a carrier for the metal surface acting as an electrode, wherein the carrier is connected to the electrode over the entire electrode surface.
An electrostatic sound transducer described in European publication No. 140,143 could be considered similar to the transducer according to the present invention as far as the external arrangement is concerned. However, this similarity does not actually exist because, in the prior art sound transducer, the diaphragm which is irradiated with sound consists of a stretchable plastics material foil with electrically conductive particles placed in the foil. This plastics material foil is mounted between two surface electrodes which are provided with holes and coated with an electrically insulating sintered material having a puncture strength of more than 3 KV.
It is, therefore, the object of the present invention to avoid the disadvantages which usually occur when a thin metal plate is used as the base plate. The invention is to make possible that metal plates can be used which have a minimum thickness of less than 0.1 mm without negatively influencing the plane parallelism between diaphragm and base plate.